KEY <XNCEPTS
Figure 1-8 shows hOil the message sent by the GOP process might be organized. It oonsists of two parts, an object reference part and a text part. The object references are for the text part of the object, the 432 port at which the process will wait for the message to be returned, arrl a reference for the process itself (GOP or IP). This last reference is not strictly necessary in the example, but is provided to shOil one way in which a message may identify its originator.
The text part of the message oontains a command field which specifies what is to be done (e.g., print one page), a status field which reflects the disposition of the print request, and the data to be printed.
With the exception of the status information, all data in the message is provided by the GOP process i the status field is updated by the printer task.
The next three sections descr ibe the operation of the example system as seen by the GOP process, the printer task, and the IP controller. These descriptions present an overview of the operations. For nnre detail on hOil these activi ties· relate to IP facilities, please refer to Appendix F, (Interprocess Cbmmunication Example), which refines the printer example.
GOP Process Perspective
To direct outp.1t to the line printer, a GOP process builds a print object and serrls it as a message to the print_request..J?Ort. The port is the process's "oonnection" to the line pr inter. After it has sent the message, the process is free to continue running. When it cannot proceed further without acknowledgement of the print operation, the process attempts to receive a message fran the print_replY'-port it specified in the print_object. When the operation has been oompleted, the process will receive the message. It then inspects the status field and takes appropriate action, perhaps writing new data into the print_object and serrling it off again.
